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What is Sourdough?

Commercial yeasts include just one kind of microorganism and one kind of yeast (Saccharomyces cerevisiae) which leaven the dough. However, Yeast and Lactic Acid Bacteria (LAB) are the two main types of microorganisms that make up the flora present in the sourdough.

The largest source of these microorganisms is flour itself, where one gram of flour contains a total of about 13.000 cells of wild yeast and approximately 320 cells of lactic acid bacteria. One gram of commercial yeasts include approximately 20 billion cells.

Wild Yeast

Yeast transforms simple sugars like glucose and fructose into alcohol (ethanol) and gas (carbon dioxide) during the fermentation process. Yeast is classified as “wild” because it is present in any natural environment. Most wild yeast cells are members of the Saccharomyces cerevisiae family, the same as commercial yeast, but their genetic characteristics are slightly different. Other species of wild yeast, such as Saccharomyces exiguus, Candida tropicalis, and Hansenula anomala have also been identified. Generally speaking, wild yeast is more resistant to acidity and temperature fluctuations compared to commercial yeast, making it better adapted to the sourdough process.

Bacteria

Lactic bacteria are part of the “bacillus” family (Lactobacillus) or “coque” family (Lactocoque) and are divided in two types: homofermentative and heterofermentative. Each has different morphology and a different reaction in the dough. Lactic bacteria also work on certain sugars, converting them into organic acids that are transformed into aromas. Two main types of organic acid produced are lactic and acetic. Lactic acid plays a direct role in bread flavor, whereas acetic acid seems to reinforce the flavor provided by the other aromas and accentuates the acid flavor of the final product with a much sharper flavor. Homofermentative bacteria produce only lactic acid; heterofermentative bacteria produce lactic acid, acetic acid, and carbon dioxide.

Source

Michel Suas (2009), Chapter 4: Fermentation. In: Advanced Bread and Pastry – A Professional Approach, Delmar Cengage Learning New York

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